Method of and means for measuring microwave power



May 3l, 1949. w. D. HERsHBl-:RGER

METHOD OF AND MEANS FOR MEASURING MICROWAVE POWER Filed May 29, 1944 :Summer RSHBER GER l llllLLlnm [1HE termined microwave frequencies.

Patented May 31, 1949 METHOD F AND MEANS FOR MEASURING MICROWAVE PGWEB William D. Hershberger, Princeton, N. J., assignor to Radio Corporation of America, a corporation of Delaware Application May 29, 1944, Serial No. 537,930 9 Claims. (Cl. 171--945) This invention relates generally to microwave transmission and more particularly to improved methods of and means for measuring power in microwave transmission systems.

The invention utilizes the characteristics of certain gases which are substantially perfect dielectrics 'at most radio frequencies but which absorb considerable energy at certain other prede- For example, in an article by Cleeton and Williams in Physical Review 45, 234 (1934) observations on microwave absorption in ammonia gas indicated that radiation having a wavelength of 1.25 centimeters will lose approximately 63 percent of its initial energy upon passing through 1.1 meters of ammonia gas 1n a non-metallic container at atmospheric pressure. It was noted further that the absorption frequency band is relatively wide since the absorption coefficient falls to approximately 1/ its maximum value at wavelengths of 1 centimeter and 1.5 centimeters. 'I'he observations described in the article identified heretofore were inspired by much earlier general theoretical work on the energy levels of the ammonia molecule together with observations on the infra red spectrum of this gas, but in al1 such prior experiments no attempt was made to determine, explain or utilize the eiect upon the gas of the microwave absorption by said gas.

'I'he instant invention provides a convenient and efficient means for utilizing the selective absorption of microwaves by ammonia and other predetermined gases, wherein the gas is enclosed within a cavity resonator which is coupled to the microwave transmission system in which the power is to be measured. At the critical irequency or frequencies at which the particular gas employed absorbs microwave energy, applicant has found the absorbed energy to be converted to heat which provides an increase in the gas pressure within the cavity resonator which may be indicated by means of any well known gas pressure indicating device. For example, the change, or rate of change, of the relative heights of the arms of a liquid enclosed within a U tube opening into the cavity resonator will provide an indication of the pressure or change of pressure within the resonator which may be calibrated in terms of the microwave power absorbed by the gas. It is believed that the temperature change in the gas due to the selective dissipation therein of microwave energy is the result of molecular resonance eilects due to excitation of the energy levels of the gas molecules. The microwave en- 2 ergy absorption in the gas proper increases as the gas pressure is increased.

Among the objects of the invention are to provide an improved method of and means for measuring microwave energy. A further object is to provide a novel method of and means for raising the temperature of a gas by selective irradiation thereof by microwave energy. Another object of the invention is to provide an improved method of and means for measuring the microwave power in a microwave transmission system. Another object of the invention is to provide an improved method of and means for subjecting a gas -tov'selective energy absorption of microwave energy to vary the temperature of the gas as a function of the microwave energy absorbed thereby. A further object of the invention is to provide an improved microwave measuring instrument comprising a gas-filled cavity resonator coupled to a source of microwave energy to be measured and means for indicating the variation in temperature of the enclosed gas as a function of the microwave energy absorbed thereby.

Other objects of the invention include an improved means for tuning a microwave cavity resonator. Another object of the invention is to provide an improved tuned cavity resonator having discrete resonant modes. An additional object of the invention is to provide an improved untuned cavity resonator having a plurality of substantially overlapping resonant modes.

The invention will be described by reference to the accompanying drawing of which Figure 1 is a cross-sectional elevational view of`one embodiment thereof, Figure 2 is a. cross-sectional elevational view of a second embodiment thereof and Figure 3 is a fragmentary cross-sectional elevational view of a modification of said first and second embodiments. Similar reference characters are applied to similar elements throughout the drawing.

Both tuned and untuned cavity resonators into which predetermined microwave energy absorbent gases may be introduced at predetermined pressures, may be employed for the measurement of microwave energy in a microwave transmission system. For the purpose of illustration, it will be assumed that the cavity resonator is filled with ammonia gas. However, various other types of gases which absorb energy in the microwave frequency range will be listed hereinafter. In the tuned cavity resonator type ofapparatus illustrated in Figure 1, the microwave eld to which the ammonia gas is subjected conforms to the 3 customary modes found in relatively sharply tuned cavity resonators, and is of high intensity since substantially all of the applied microwave energy is absorbed in the resonator. 'I'his is accomplished by tuning the cavity resonator to the applied frequency by means of a tuning plunger disposed within a ,portion of the resonant chamber which is coupled through a microwave permeable window to the gas-tight portion of the resonator enclosing the gas. A properly dimensioned input iris bounds a second microwave permeable window which opens into the transmission waveguide. Reactive tuning elements coupled to the waveguide intermediate the source of microwave energy and the cavity resonator provide proper matching of the resonator to the transmission system substantially to prevent wave reflections and thereby to insure that all transv mitted energy is confined to the cavity resonator and thereby absorbed by the gas therein.

An untuned cavity resonator of the type illustrated in Figure 2 is proportioned so'that, in view of the Q of the device-which is determined by the resonator wall losses and the losses in the ammonia gas-the resonant modes are so closely spaced as to overlap. This condition may be achieved by selecting the volume of the resonator to be larger than some minimum value in view of the expected Q of the resonator. The number of resonant modes An lying in the frequency range Af, which is determined in turn by Athe value of Q in the relation 'smi y, f is approximately 8f V0 (l) An- Q where p is the static pressure and T is the ab-V solute temperature. A

It should be understood that the variation in temperature, and hence in the pressure of the gas within the cavity resonator in response to absorbed microwave energy, will be a function of the energy absorbed by the gas, the wall losses of the cavity resonator, and the heat transferred from the gas to the cavity resonator walls. The energy directly absorbed by the gas from the microwave transmission system provides a substantially rapid increase in gas temperature and pressure since the gas has a relatively low heat capacity and a relatively high temperature coefficient of expansion. These features therefore will provide a relatively rapid rise in the gas pressure indicated by the gas pressure indicator. However, unless the cavity resonator walls are ythermally insulated from the enclosed gas, or are maintained at substantially constant temperature, the heat transfer between the cavity walls and the enclosed gas will provide a relatively slow protracted pressure variation in the enclosed gas which wil1 provide spurious indications of the applied microwave energy. The ratio of the energy dissipated in the gas proper to the energy dissipated in the resonator walls is a function of the initial pressure of the gas in the resonator.

As will be explained hereinafter, spurious indications due to temperature variations in the cavity resonator walls may be substantially eliminated by lining the cavity resonator walls with a thermal insulator which effectively minimizes heat transfer between the walls and the gas. Also, if desired, the cavity resonator walls may 'be subjected to an air blast to maintain them at substantially constant temperature during the measurement period of the apparatus.

It should be understood that the cavity resonator, when properly matched to the transmission waveguide, performs substantially as a perfectly matched load which absorbs all of the microwave energy introduced thereto. Therefore, the wattmeter constructed in this manner actually absorbs all microwave energy applied to the device.

Frequently it is desirable to measure the microwave energy continuously applied to an auxiliary device. Under these conditions the power measuring apparatus comprising the wattmeter should be as efficient as possible, or alternatively, it should absorb only a predetermined fixed portion of the microwave energy transmitted from the microwave generator to the auxiliary load device. This feature may be accomplished by connecting the cavity resonator wattmeter to the main transmission waveguide through a branched waveguide which is coupled to the main wave'- guide 'through' a xed aperture device providing predetermined xed couplingl to the wattmeter. Initially, the wattmeter coupling factor may be determined by measuring separately the actual power delivered to the-auxiliary load and to the wattmeter. Such a periodic coupling provides a substantially uniform energy transfer to the wattmeter over a relatively wide frequency range, thereby making the wattmeter coupling element substantially independent of operating frequency.

Referring to Figure 1 of the drawing, a waveguide' I, which may be of the conventional rectangular cross-section, includes branched waveguides 3, 5, having tuning pistons disposed therein. The pistons may be longitudinally adjusted by means of tuning knobs l, 9. respectively, for introducing desired reactances into the waveguide I for matching the wattmeter to the waveguide. One end of the waveguide I includes a flange II which may be screwed to a complementarily flanged collar I3 having a threaded portion I5 adapted to engage a complementarily-1 threaded aperture in one end of a tuned cavity? resonator wall I1.

The cavity resonator comprises a relatively heavy metallic block having a central opening communicating at opposite ends with relatively large threaded apertures for engaging the threaded portion I5 of the collar I3 and a threaded portion I9 of a tuning cylinder 2|, respectively.

The tuning cylinder 2i has an internal aperture coaxial and substan-tially coexten'sive with the internal aperture of the cavity resonator. The end of the tuning cylinder 2| remote from the threaded portion I9 thereof engaging the cavity resonator is terminated by a -threaded apertured plug 23 which engages a coaxial complementarily threaded tuning shaft 25 terminated by a third tuning knob 21. Rotation of Ithe third tuning knob 21 and the tuning shaft 25 within the complementarily threaded plug 23 r varies the longitudinal position of a tuning plug a manner well known in the art. A split key 35 secured to the outer end of the tuning plug 29 engages a complementary undercut shoulder near the inner end of the tuning shaft to permit rotation thereof without relatively longitudinal displacement of the tuning shaft and the tuning plunger, whereby rotation of the tuning shaft provides longitudinal adjustment of the tuning plunger 29 within the tuning cylinder 2|.

A coupling aperture plate juxtaposed with a microwave permeable window 31, both of which are sealed to the walls of the cavity resonator |1 by means of a compressible rubber gasket 39, are held in place by the threaded end portion I5 of the collar i3 to provide an effective gas seal between the cavity resonator and the waveguide I. Similarly, a second microwave permeable window 4| and a second compressible rubber gasket 43 held in place by the threaded end portion I5 of the tuning cylinder 2| permit tuning of the resultant gas-tight cavity occupying the space between the microwave permeable windows 31. 4|, .by means of the externally disposed tuning plug An intake aperture 45 in the side/wall of the cavity resonator I1 communicates with a source of microwave energy absorbent gas, not shown, by means of a pipe 41 sealed into the side wall of the cavity I1. A control valve 49 is interposed in the pipe 41 to control the admission of the gas into the gas-tight cavity resonator. A vent valveI 5I, of any well known type which may be opened and closed at will, permits expulsion of undesired gases from the interior of the cavity resonator when the valve 49 is opened to admit the desired microwave absorbent gas. y

Another aperture 53, in the inner wall of the cavity resonator.- permits the attachment of a conventional U-shaped capillary tube 55, one end of vwhich maybe sealed into the cavity resonator wall by means of a gas-tight collar 51. The lower portion of the `utube 55 is filled with a colored inert uid column 59, the height of which may be employed to indicate the gas pressure or changes in pressure within the cavity resonator. The remote end of the U-tube vopens into an enlarged expansion chamber 5| which also comi is opened first -and then, while the vent-valve 5| is held open, the first valve 49 is opened to admit ammonia gas until all of the air originally in the cavity resonator is expelled through the vent- 6 y resonator as well as within the expansion chamber 5|. Both ilrst and second valves 49, t9 then are closed in the order named.

When microwave energy is coupled through the input aperture plate 95 from the waveguide I into the cavity resonator, the energy will be absorbed by the ammonia gas within the resonator, raising its temperature and hence its pressure. The variation in gas pressure thereby occurring on only one side of the U-tube 55 will vary the height of the enclosed fluid column 59 with respeet to the fixed scale '55.

The cavity resonator may be tuned to resonance with the applied microwave energy by adjustment of the third control knob 21 which longitudinally displaces the tuning plug 29. With fixed microwave input the third tuning knob 21 is adjusted until maximum displacement of the liquid column 59 is obtained.

The cavity resonator impedance may be matched to the surge impedance of the waveguide i, to eliminate substantially al1 wave reflections therefrom back into the waveguide, by adjustment of the control knobs 1, 9, of the reactive waveguide tuning stubs 3. 5. respectively. Thus the cavity resonator will absorb substantially all microwave energy transmitted by the waveguide I by providing a substantially perfect termination therefor. The energy absorbed by the cavity resonator and the ammonia gas enclosed therein will be dissipated in the form of heat generated directly in the ammonia gas and as heat generated in the cavity resonator walls due to the electrical resistance thereof.

As explained heretofore, the most desirable operating condition is to segregate as much as possible the heating effects -upon .fthe enclosed gas from the heating eilects upon the cavity resonator walls. This may be accomplished by lining the interior lwalls of the resonant cavity with a layer of felt 51. or other thermal insulating materi-al, Ito prevent effective heat transfer between the gas and the inner walls of the cavity resonator.

It should be understood that the reactive matching stubs 3, -5 coupled to the input Waveguide I may be omitted, and any other type of reactive tuning devices known in the microwave art may be substituted therefor which will pro-` vide effective matching of the impedances of the waveguide I and the cavity resonator.

The electromagnetic field established within the cavity resonator will provide at least two regions of maximum flux density, since the tun`y ing plug 29 is adjusted to provide an eiectivei cavity resonator length of one wavelength at the operating frequency. Since the resonant characteristics of the gas enclosed within the cavity resonator are not critical with respect to frequency, the adiustment of the tuning plunger 29. need not be especially critical, and the pressure response of the enclosed gas will be substantially independent of relatively small changes in the frequency of the applied microwave energy.

Figure 2 shows an untuned cavity resonator wattmeter connected to terminate a branch waveguide which is coupled to a main transmission waveguide 1I through a limiting aperture device i tion of the waveguides I and 1| to divert a prevalve 5I. The vent-valve 5| then is closed, and

the valve 49 is left open until the desired pressure determined portion of the microwave energy in the main waveguide 1| to the wattmeter. As` explained heretofore, the aperture device position of ammonia Sas is attained within the cavity 7 5 and proportions may be calculated in a manner wave power transmitted through the waveguide,

I to the wattmeter. It should be understood that lthis method of coupling the wattmeter to the 'microwave source to be measured also may be employed in the embodiment of the invention described and illustrated with respect to Figure 1.

The end of the wattmeter -waveguide I remote from the main transmission waveguide 1| is terminated in a conductive flange II which is screwed to a complementary conductive flange vprojecting from one end of theuntuned watt- `meter cavity resonator 11. The untuned cavity resonator 11 may be proportioned, as described in detail heretofore, in order to provide a relatively large number of overlapping resonant modes, the resultant of which provides relatively wide frequency band response. The opening through the flange 15 of the cavity resonator. .-11 is covered by a microwave permeable window y31, the edges of which are gas-sealed by means of a compressible rubber gasket 'ring 39. The window 31 and gasket 39 are maintained under compression by means of an externally threaded annular conductive ring 19 which is threaded to .the inner side of the end of the cavity resonator 11. The entire inner surface of the cavity res- -onator 11 may be thermally insulated by means of 'an insulating layer 61 of felt or other mate- ;rial, as described heretofore with respect to the cavity resonator of Figure 1.

The impedances of the cavity resonator 11 and the wattmeter waveguide I may be substantially matched, to prevent wave reflections from the wattmeter, 'by means of three reactive tuning screws 8l, 83, 85 disposed within a section 81 of the waveguide I intermediate the limiting aperlture device 13 and the cavity resonator 11.

The system for introducing the microwave absorptive gas, venting undesired gases, and measuring gas pressure variation in the cavity resonator due to absorbed microwave energyv may be of the type described heretofore in Figure 1 or .any other type known in the art. It should be understood that, in the untuned type of cavity resonator having a large number of resonant modes, the microwave flux distribution-within the cavity resonator will include a relatively large number of regions of high flux density distribution more or less uniformly spaced throughout the cavity resonator interior. Since the cavity resonatorimpedance is substantially matched t0 the wattmeter transmission waveguide I, the res- `tion therefor which absorbs all of the microwave energy introduced thereto. Also, since the pro- .portions of the total microwave energy derived.

from the microwave source, transmitted by the main waveguide 1I and diverted to the watt- .meter 11 are known, the indications provided byA guide for use in providing the power indications.

Figure 3 discloses an alternative thermal insulating structure which may be employed to ther- -xnally insulate the inner walls of either of the '1.15,

onator provides a substantially perfect termina'- l tus of the type described heretofore.

paratus employed heretofore.

cavity resonators of the devices of Figures 1 and 2 from the microwave absorptive gases enclosed therein. The inner surface 89 of the cavity resonator wall may be covered by means of a double layer of glass 9|, 93 having a space 95 therebetween which is substantially evacuated'.

This type of thermal insulation, which is substantially similar to the well known Dewar flask, provides extremely Veffective thermal insulation while being extremely permeable to the microwave field within the cavity resonator.

It should be understood that the liquid selected for the liquid column 59 in the U tube indicator l should be substantially inert with respect to the particular type of microwave absorptive gas employed in the cavity resonator. For example, when employing ammonia gas, colored kerosene has been found to be satisfactory as an indicating liquid column.

Various other microwave absorptive gases have been tested and found to be quite satisfactory for microwave power measurements in appara- The following table discloses the microwave frequencies at which some of these various gases have been found to absorb considerable microwave energy as indicated by the absorption coefcientswhich have been measured:

Thus the invention described heretofore provides an improved method -of and lmeans for measuring microwave energy which differs substantially in operation from methods and ap- The devices 'disclosed provide extremely convenient and accuratev means for measuring microwave energy in the .millimeter and centimeter wave region, since I the measurements are dependent upon heat which is generated directly within a microwave absorptive gas which also is employed as the thermometric element of the power indicator.

, Since the thermal capacity of most gases is extremely low and the thermal coeflicient of expansion is relatively high, the response of the system to applied microwave energy is relatively *sensitive and rapid. l

Thus thegas responsive type of wattmeter has the following advantages v over wattmeters of the bolometer or circulating -liquid types in present use: (1) swift response to applied microwave energy,-Y (2) substantially greater sensitivity to frequency range in which present types of microwave power measuring apparatus are inefcient.

. I claim as my invention:

1. The method of measuring microwave energy with apparatus including a closed gas chamber comprising dissipating substantially all of said energy in said chamber and said enclosed gas to establish changes in the molecular venergy levels in said gas subjected to normally fixed pressure and temperature conditions within said gas chamber to provide expansion of said gas, tuning said chamber to electrical resonance with said microwave energy, and indicating said expansion of said gas in terms of said microwave energy applied thereto and absorbed thereby.

2. The method of measuring microwave energy with apparatus including a gas chamber comprising dissipating substantially all of said energy in said gas to establish changes in the molecular energy levels in said gas subjected to normally fixed pressure and temperature conditions Within said gas chamber to provide expansion of said gas, tuning said chamber to electrical resonance with said microwave energy, electrically matching said chamber to a line transmitting said energy, and indicating said expansion of said gas in terms of said microwave energy applied thereto and absorbed thereby.

3. A wattmeter for microwave energy including means defining a closed conductive chamber, a gas disposed within said chamber under normally iixed pressure and temperature conditions, microwave permeable windows in said chamber, means for introducing said energy into said chamber through one of said windows to dissipate substantially all of said energy in said gas with resultant expansion of said gas, means disposed externally of said chamber for electrically tuning said chamber through another of said windows to resonate said chamber to said microwave energy, and means for indicating said expansion of said gas in terms of said dissipated energy. l

4. Apparatus of the type described in claim 3 including means for matching the impedance of said chamber to a source of said energy to efectlvely minimize wave reflections from said,

chamber.

5. A wattmeter for microwave energy including closed thermally insulated conductive means for enclosing a microwave energy absorptive gas, means for dissipating in said gas substantially all of said energy to be measured to excite a change in the molecular energy levels in said gas with resultant expansion thereof, means for indicating said expansion of said gas in response to said molecular energy level changes in terms of the microwave power absorbed by said gas, and means for electrically matching the impedance of said gas enclosing means to a line transmitting the microwave energy to be measured.

6. A wattmeter for microwave energy including closed thermally regulated conductive means for enclosing a microwave energy absorptive gas, means for dissipating in said gas substantially all of said energy to be measured to excite a change in the molecular energy levels in said gas with `resultant expansion thereof, means for indicating said expansion of said gas in response to said molecular energy level changes in terms o the microwave power absorbed by said Zas. anI

means for electrically matching the impedance of said gas enclosing means to a line transmitting the microwave energy to be measured.

7. A wattmeter for microwave energy including means defining a. closed conductive chamber, ammonia gas disposed in said chamber under normally fixed pressure and temperature conditions, means for dissipating substantially all of said energy in said gas to vary the pressure thereof due to changes in the molecular energy levels established in said gas in response to said dissipated energy. means for indicating said pressure variations in terms of said dissipated energy, and means external of said chamber for tuning said chamber to resonate to said microwave en ergy.

8. Apparatus of the type described in claim 7 including means for matching the impedance of said chamber to a source of said energy to effectively minimize wave reflections from said chamber.

9. A wattmeter for microwave energy including means defining a closed conductive chamber, a gas disposed within said chamber under normally fixed pressure and temperature conditions, a microwave permeable window in said chamber, means for introducing said energy into said chamber to dissipate substantially all of said energy in said gas with resultant expansion of said gas,

means disposed externally of said chamber for electrically tuning said chamber through said window to resonate said chamber to said microwave energy. and means for indicating said expansion of said gas in terms of said dissipated energy.V

WILLIAM D. HERSHBERGER.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 456,172 Thomson July 21, 1891 763,164 Donitz June 21, 1904 850,065 Shoemaker Apr. 9, 1907 1,900,573 McArthur Mar. 7, 1933 1,940,759 Lincoln Dec. 26, 1933 2,151,118 King et al Mar. 21 1939 2,179,261 Keller Nov. 7, 1939 2,212,211 Pfund Aug. 20. 1940 2,241,119 Dallenbach May 6, 1941 2,241,976 Biewett et al May 13, 1941 2,262,020 Llewellyn Nov. 11, 1941 2,398,606 Wang Apr. 16, 1946 2,400,777 Okress May 21, 1946 2,405,841 Brannin a Aug. 13, 1946 OTHER REFERENCES Publication in Physical Review, vol. 45, Page 234 (1934) by Cleeton and Wiliams. (Copy in Patent Omce Library.) 

